The problem of Islamist radicalisation within prisons was highlighted by the Westminster attacks. Khalid Masoon is understood to have converted to an extremist sect of Islam while in HMP Lewes, East Sussex.

Michael Adebolajo – the murderer of fusilier Lee Rigby – was also recently transferred to Frankland, due to concerns he was radicalising prisoners in HMP Belmarsh.

In his report for the UK government, former prison governor Ian Acheson suggested forming segregation units to hold prisoners posing "particular and enduring risk to national security" in eight of the UK's most secure jails. Each unit would hold up to 50 prisoners.

But prison officers have expressed concerns over the plan, warning that separating terrorists out could give them elevated status among prisoners.

Steve Gillan, general secretary of the Prison Officers Association (POA), told the Guardian: "We saw it in Northern Ireland, where some loyalist prisoners and some republican prisoners were segregated and what happens was that it gave them a political status. It didn't work and in fact made the situation worse."

Gillan went on to warn that some prisoners may engage in "bad behaviour" in order to be transferred to the special unit, which would make prison officers' work harder.

Jackie Marshall, the POA's representative for the special units and member of the union's national executive committee, has denied that the special units are designed to segregate prisoners.

"The unit in Frankland is a separation unit, it is not a segregation unit," she said, "because the prisoners who will be in it are not being punished – they are being held separately."

Roberta Blackman-Woods, MP for Durham City, supports the unit's creation in Frankland. In a statement quoted by the Guardian she said, "If this unit can help in the wider deradicalisation and Prevent strategy, then I think it's a good thing and it's a recognition that there's an issue that needs to be tackled."

Methods of radicalisation

Ian Acheson's report listed some of the ways Islamists may try to recruit other prisoners:

Gang culture and the violence, drug trafficking and criminality directed by extremists

Convicted terrorists advocating support for Isis, and threatening staff and prisoners

Charismatic prisoners acting exerting a controlling and radicalising influence on the wider prison population

Books and materials promoting extremism available in chaplaincy libraries or held by individual prisoners;

Intimidation of prison imams

Exploitation of staff's fear of being labelled racist

Abuse of a prisoner's right to confidential written communication with their legal representatives.

Source: Summary of the main findings of the review of Islamist extremism in prisons, probation and youth justice, Ministry of Justice, National Offender Management Service

A Ministry of Justice spokesperson told the Guardian: "Islamist extremism is a danger to society and a threat to public safety – it has to be defeated wherever it is found. We are committed to confronting and countering the spread of this poisonous ideology in prisons.

"Preventing the most dangerous extremists from radicalising other inmates is essential to the safe running of our prisons and fundamental to public protection."

The pilot scheme at Frankland follows Acheson's warning that Islamist terrorists could be recruiting within UK jails.

Acheson's report raised concerns that the high number of people being convicted of terrorism offences means they can no longer be held in highest security prisons or wings, due to a lack of capacity, and must be housed in the general population.

"Such prisoners extend the threat of radicalisation beyond those arrested for terrorist offences," said a Ministry of Justice summary of Acheson's report. "Other prisoners – both Muslim and non-Muslim – serving sentences for crimes unrelated to terrorism are nevertheless vulnerable to radicalisation by Islamist extremists."

Government figures reported that as of 31 December 2015, there were 85,000 prisoners in England and Wales compared to 83,400 in 2009. Of these new prisoners, 24% identified as Muslim, leading to a rise of 2.6% in the numbers who claim to follow Islam. The latest government figures show there are approximately 85,500 prisoners held in the UK.